Selective binocular vision loss in two subterranean caviomorph rodents: Spalacopus cyanus and Ctenomys talarum.

To what extent can the mammalian visual system be shaped by visual behavior? Here we analyze the shape of the visual fields, the densities and distribution of cells in the retinal ganglion-cell layer and the organization of the visual projections in two species of facultative non-strictly subterranean rodents, Spalacopus cyanus and Ctenomys talarum, aiming to compare these traits with those of phylogenetically closely related species possessing contrasting diurnal/nocturnal visual habits. S. cyanus shows a definite zone of frontal binocular overlap and a corresponding area centralis, but a highly reduced amount of ipsilateral retinal projections. The situation in C. talarum is more extreme as it lacks of a fronto-ventral area of binocular superposition, has no recognizable area centralis and shows no ipsilateral retinal projections except to the suprachiasmatic nucleus. In both species, the extension of the monocular visual field and of the dorsal region of binocular overlap as well as the whole set of contralateral visual projections, appear well-developed. We conclude that these subterranean rodents exhibit, paradoxically, diurnal instead of nocturnal visual specializations, but at the same time suffer a specific regression of the anatomical substrate for stereopsis. We discuss these findings in light of the visual ecology of subterranean lifestyles.

Can red flowers be conspicuous to bees? Bombus dahlbomii and South American temperate forest flowers as a case in point.

It has been argued that trichromatic bees with photoreceptor spectral sensitivity peaks in the ultraviolet (UV), blue and green areas of the spectrum are blind to long wavelengths (red to humans). South American temperate forests (SATF) contain a large number of human red-looking flowers that are reported to be visited by the bumblebee Bombus dahlbomii. In the present study, B. dahlbomii's spectral sensitivity was measured through electroretinogram (ERG) recordings. No extended sensitivity to long wavelengths was found in B. dahlbomii. The spectral reflectance curves from eight plant species with red flowers were measured. The color loci occupied by these flowers in the bee color space was evaluated using the receptor noise-limited model. Four of the plant species have pure red flowers with low levels of chromatic contrast but high levels of negative L-receptor contrast. Finally, training experiments were performed in order to assess the role of achromatic cues in the detection and discrimination of red targets by B. dahlbomii. The results of the training experiments suggest that the bumblebee relies on achromatic contrast provided by the L-receptor to detect and discriminate red targets. These findings are discussed in the context of the evolutionary background under which the relationship between SATF species and their flower visitors may have evolved.

A simple method to microinject solid neural tracers into deep structures of the brain.

We have developed an instrument to perform microinjections of solid neural tracers into deep structures of the brain. The instrument consists of a thin hypodermic needle equipped with a movable internal rod, which is connected to a pressure chamber. When a pressure pulse is applied to the chamber, the rod moves forward and back inside the needle, pushing out a solid load previously packed inside the needle tip. By attaching a microelectrode to the instrument, it is also possible to have electrophysiological control of the injection placement. To test the instrument, we microinjected DiI and rhodamine crystals into selected structures of the visual system of pigeons. The results show small, well-defined injection sites, accurately located in the desired targets, together with well-developed anterogade and retrograde transport, selectively originated from the injection sites. This method extends the usage of solid tracers to most structures in the brain and may, in certain cases, be more advantageous than the conventional method of injecting tracer solutions.

Spatiotemporal profile of synaptic activation produced by the electrical and visual stimulation of retinal inputs to the optic tectum: a current source density analysis in the pigeon (Columba livia).

The optic tectum of the pigeon is a highly organized, multilayered structure that receives a massive polystratified afference of at least five different populations of retinal ganglion cells and gives rise to various anatomically segregated efferent systems. The synaptic organization of retino-tectal circuitry is, at present, mostly unknown. To investigate the spatiotemporal profile of synaptic activation produced by differential (electrical and visual) stimulation of the retinal inputs, we performed a high-spatial-resolution current source density analysis in the optic tectum of the anaesthetized pigeon. Electrical stimuli consisted of brief pulses of different durations applied to the optic nerve head, while visual stimuli consisted of light flashes of different intensities. Electrical stimulation generated sinks confined to retinorecipient layers. The temporal structure, spatial location and thresholds of these sinks indicated that they are all due to primary tectal synapses of retinal fibers with different conduction velocities. Sinks evoked by the fastest retinal axons were more superficially located than sinks produced by slower retinal fibers. Visual stimulation, on the other hand, resulted in a more complex pattern of current sinks, with various sinks located in the retinorecipient layers and also well below. Visual stimulation induced action potentials at superficial as well as deep tectal levels. We conclude that electrical stimulation activates most of the populations of ganglion cells as well as their primary tectal synapses, but is unable to elicit a significant activation of secondary tectal synapses. Visual stimulation, on the contrary, activates just some of the incoming retinal populations, but in a way that produces noticeable secondary activation of intratectal circuits. Laminar segregation of retinally evoked tectal activity, as reported here, has also been found in other vertebrates. Similarities and differences with previous studies are discussed.

Reversible mechanical fixation of eye position in awake head-restrained pigeons (Columba livia).

Here we describe a method to fix gaze positions and to significantly reduce saccadic oscillations in pigeons. The procedure consists of a mechanical immobilization of the eye through the use of an electromagnet that exerts a radial force upon a small metal rectangle glued to the dorsal part of the eye. The method can be used in avian visual neurophysiology in order to hold the eye immobilized for periods of time, long enough to map the properties of visual receptive fields and investigate the possible functions of saccadic oscillations.

Two distinct populations of tectal neurons have unique connections within the retinotectorotundal pathway of the pigeon (Columba livia).

The tectofugal pathway is a massive ascending polysynaptic pathway from the tectum to the thalamus and then to the telencephalon. In birds, the initial component of this pathway is known as the tectorotundal pathway; in mammals, it is known as the tectopulvinar pathway. The avian tectorotundal pathway is highly developed; thus, it provides a particularly appropriate model for exploring the fundamental properties of this system in all amniotes. To further define the connectivity of the tectorotundal projections of the tectofugal pathway, we injected cholera toxin B fragment into various rotundal divisions, the tectobulbar projection, and the ventral supraoptic decussation of the pigeon. We found intense bilateral retrograde labeling of neurons that stratified within layer 13 and, in certain cases, granular staining in layer 5b of the optic tectum. Based on these results, we propose that there are two distinct types of layer 13 neurons that project to the rotundus: 1) type I neurons, which are found in the outer sublamina of layer 13 (closer to layer 12) and which project to the anterior and centralis rotundal divisions, and 2) type II neurons, which are found in the inner sublamina of layer 13 (closer to layer 14) and which project to the posterior and triangularis rotundal divisions. Only the labeling of type I neurons produced the granular dendritic staining in layer 5b. An additional type of tectal neuron was also found that projected to the tectobulbar system. We then injected Phaseolus vulgaris-leucoagglutinin in the optic tract and found that the retinal axons terminating within tectal layer 5b formed narrow radial arbors (7-10 microm in diameter) that were confined to layer 5b. Based on these results, we propose that these axons are derived from a population of small retinal ganglion cells (4.5-6.0 microm in diameter) that terminate on the distal dendrites of type I neurons. This study strongly indicated the presence of a major bilateral oligosynaptic retinotectorotundal pathway arising from small retinal ganglion cells projecting to the rotundus with only a single intervening tectal neuron, the proposed type I neuron. We suggest that a similar organization of retinotectopulvinar connections exist in reptiles and in many mammals.

GABAergic inputs to the nucleus rotundus (pulvinar inferior) of the pigeon (columba livia).

The avian nucleus rotundus, a nucleus that appears to be homologous to the inferior/ caudal pulvinar of mammals, is the major target of an ascending retino-tecto-thalamic pathway. Further clarification of the inputs to the rotundus and their functional properties will contribute to our understanding of the fundamental role of the ascending tectal inputs to the telencephalon in all vertebrates, including mammals. We found that the rotundus contains a massive plexus of glutamic acid decarboxylase (GAD)-immunoreactive axons using antibodies against GAD. The cells within the rotundus, however, were not immunoreactive for GAD. The retrograde tracer cholera toxin B fragment was injected into the rotundus to establish the location of the afferent neurons and determine the source of the gamma-aminobutyric acid (GABA) inputs into the rotundus. In addition to the recognized bilateral inputs from layer 13 of the tectum, we found intense retrograde labeling of neurons within the ipsilateral nuclei subpretectalis (SP), subpretectalis-caudalis (SPcd), interstitio-pretecto-subpretectalis (IPS), posteroventralis thalami (PV), and reticularis superior thalami (RS). All the neurons of the SP, SPcd, IPS, and PV were intensely GAD-immunoreactive. The neurons of layer 13 of the tectum were not immunoreactive for GAD. Following the destruction of the ipsilateral SP/IPS complex, we found a major reduction in the intensity of the GAD axonal immunoreactivity within the ipsilateral rotundus, but this destruction did not diminish the intensity of the GAD-immunoreactivity within the contralateral rotundus. Our studies indicated that the source of the massive GAD-immunoreactive plexus within the rotundus was from the ipsilateral SP, SPcd, IPS, and PV nuclei. These nuclei, in turn, received ipsilateral tectal input via collaterals of the neurons of layer 13 in the course of their projections upon the rotundus. We suggest that the direct bilateral tecto-rotundal projections are excitatory, whereas the indirect ipsilateral projections from the SP/IPS and PV are mainly inhibitory, possibly acting via a GABA-A receptor.

Conduction velocity groups in the retino-tectal and retino-thalamic visual pathways of the pigeon (Columbia livia).

The anatomical characteristics of the avian visual system are well known. However, there are wide gaps in our knowledge with respect to the physiological characteristics of their visual system. For example, we lack both an operational identification of the different ganglion cell types present in the retinae of birds, and a description of their presumptive differential central projections. The results presented here address this latter point by classifying the conduction velocity groups of fibers present in the optic tract of the pigeon. We report the existence of at least 5 groups of axons in the optic tract of the pigeon, with conduction velocities of 22-18 m/s, 12-10 m/s, 8 m/s, 6 m/s and less than 2.5 m/s. All five groups project to the tectum but only the four fastest groups project to the dorsal thalamic complex. The homologies with the populations of retinal axons found in cats are discussed.

Brain, language and the origin of human mental functions.

We propose that to understand the biological and neurophysiological processes that give rise to human mental phenomena it is necessary to consider them as behavioral relational phenomena. In particular, we propose that: a) these phenomena take place in the relational manner of living that human language constitutes, and b) that they arise as recursive operations in such behavioral domain. Accordingly, we maintain that these phenomena do not take place in the brain, nor are they the result of a unique operation of the human brain, but arise with the participation of the brain as it generates the behavioral relational dynamics that constitutes language.

Linear analysis of auto-organization in Hebbian neural networks.

The self-organization of neurotopies where neural connections follow Hebbian dynamics is framed in terms of linear operator theory. A general and exact equation describing the time evolution of the overall synaptic strength connecting two neural laminae is derived. This linear matricial equation, which is similar to the equations used to describe oscillating systems in physics, is modified by the introduction of non-linear terms, in order to capture self-organizing (or auto-organizing) processes. The behavior of a simple and small system, that contains a non-linearity that mimics a metabolic constraint, is analyzed by computer simulations. The emergence of a simple "order" (or degree of organization) in this low-dimensionality model system is discussed.

Effects of retinal lesions upon the distribution of nicotinic acetylcholine receptor subunits in the chick visual system.

Immunohistochemistry was used in this study to evaluate the effects of retinal lesions upon the distribution of neuronal nicotinic acetylcholine receptor subunits in the chick visual system. Following unilateral retinal lesions, the neuropil staining with an antibody against the beta 2 receptor subunit, a major component of alpha-bungarotoxin-insensitive nicotinic receptors, was dramatically reduced or completely eliminated in all of the contralateral retinorecipient structures. On the other hand, neuropil staining with antibodies against two alpha-bungarotoxin-sensitive receptor subunits, alpha 7 and alpha 8, was only slightly affected after retinal lesions. Decreased neuropil staining for alpha 7-like immunoreactivity was only observed in the nucleus of the basal optic root and layers 2-4 and 7 of the optic tectum. For alpha 8-like immunoreactivity, slight reduction of neuropil staining could be observed in the ventral geniculate complex, griseum tecti, nucleus lateralis anterior, nucleus lentiformis mesencephali, layers 4 and 7 of the tectum, and nucleus suprachiasmaticus. Taken together with previous data on the localization of nicotinic receptors in the retina, the present results indicate that the beta 2 subunit is transported from retinal ganglion cells to their central targets, whereas the alpha 7 and alpha 8 subunit immunoreactivity appears to have a central origin. The source of these immunoreactivities could be, at least in part, the stained perikarya that were observed to contain alpha 7 and alpha 8 subunits in all retinorecipient areas. In agreement with this hypothesis, the beta 2 subunit of the nicotinic acetylcholine receptors was not frequently found in perikarya of the same areas.

[Perception: behavioral configuration of the object]. / Percepción: configuración conductual del objeto.

The word perception is usually heard as connoting an operation of grasping an external reality through the process of receiving information from it. This, however, is constitutively impossible because living systems are dynamic structure determined systems, and everything happens in them determined at every instant by their structure. This means that the medium cannot specify what happens in a living system, and that it can only trigger in it structural changes determined in its structure. As a result a living system constitutively always operates in structural congruence with the medium, and exists as such only as long as this structural congruence (adaptation) is conserved; otherwise it desintegrates. In these circumstances, the phenomenon connoted by the word perception consists in the association, by the observer, of the behavioral regularities that he or she distinguishes in the observed organism with the conditions of the medium that he or she sees triggering them. The observer uses such behavioral regularities to characterize perceptual objects. This applies to all living systems including the observer. The explanation of perception in the context of the structural determinism of living systems invalidates any attempt to account for the phenomenon of cognition (including language) with notions that entail the denotation or connotation of a domain of reality independent of the distinctions of the observer.

Regional specialization of the quail retina: ganglion cell density and oil droplet distribution.

The ganglion cell density of the quail's retina was studied in sections and whole mounts. Two regions of high ganglion cell density were found, corresponding to an afoveate area centralis and an area dorsalis. Oil droplets were found to be isotropically distributed throughout the retina. It is proposed that the significance of such retinal regional specialization, in comparison to similar studies in the pigeon and the chick, is that regional specialization in the avian retina is more closely related to feeding habits than to phylogenetic descendence.